Chart: time markers stored in the archive

Why

An operator needs to mark a moment: the start of a process, an event worth returning to. Nothing in the viewer does that today.

Wanted

• A vertical line placed at a timestamp, scrolling with the chart and leaving the view when the window moves past it.
• Stored in the archive database (Semiteq/SemiBase#10), so it survives a restart and is visible from another station. Writing goes through the semiplot_writer role (Semiteq/SemiBase#8).
• Draggable to another time, deletable from a context menu.
• Every pen shows its value where the marker crosses it.

What exists to build on

The delta tool is the structural precedent and covers three of the four parts:

• Two VerticalLine plottables, created once, pinned to the shared bottom axis, hidden by default (Chart/TrendChartView.axaml.cs:330-339). They are the only vertical lines in the repository.
• Placement routed through ChartPressRouter (Chart/ChartPressRouter.cs:21-23) into a pure Core model (Core/Trends/DeltaCursorModel.cs:15-26).
• Re-projection on every window change (TrendChartView.axaml.cs:347-363).
• Value at a crossing already exists as a pure function: CursorReadoutModel.ReadAt interpolates each pen's centre and returns null in a gap or outside the range (Core/Trends/CursorReadoutModel.cs:5-30).

Genuinely new: dragging a plottable, which has no precedent since the only drag is a whole-canvas pan; a context menu, of which the repository has none; and persistence.

To settle

• How many markers, and what happens when the window holds a hundred of them.
• Whether a marker carries a label, and where it is drawn.
• Whether a marker is the bookmark of docs/architecture/trend-feature-spec.md:144-146 (CU-4, SHOULD, placed by double-click in cursor mode), or the annotation that trend-interaction.md:84,282 declares out of scope. Persistence pushes it towards the second, so the docs need a decision recorded either way.
• Whether markers reload when another station adds one, or only at startup.